The present invention relates to an exhaust gas recirculation system in an internal combustion engine, and more particularly to a control apparatus for an exhaust gas flow control valve of exhaust gas recirculation system.
Heretofore, there have been many proposals to introduce, throughout all operating conditions of an internal combustion engine, a substantially inert gas such as exhaust gas into the intake system; i.e. into the intake manifold or into the induction passage at a location downstream of the air filter; with the intention of reducing the concentration of nitrogen oxides (No.sub.x) in the engine exhaust gases by suppressing their formation. It is required that a suitable amount of exhaust gas be introduced into the intake system of the engine to attain satisfactory results. A reduction in engine performance results if the ratio of the flow rate of recirculated exhaust gas to that of intake air exceeds a certain optimum value, and failure to suppress the formation of nitrogen oxides results if the ratio is much smaller than the certain optimum value. If intake manifold vacuum is employed to actuate a valve for controlling the flow of exhaust gas into the intake system of the engine, as is common in the prior art, it is hardly possible to maintain the flow rate ratio of recirculated exhaust gas to intake air at an optimum value mostly due to the fact that the vacuum in the intake manifold does not vary in proportion to the intake air flow rate throughout operation of the engine.
It has been confirmed that the vacuum in the venturi of a carburetor varies in relation to the velocity of air flowing through the venturi throughout operation of the engine, and therefore in relation to the flow rate of intake air being introduced into the engine. Thus, if the vacuum in the venturi is employed as a variable in controlling the amount of exhaust gas introduced into the intake system, the flow rate of exhaust gas can be metered to an optimum ratio to that of intake air throughout all modes of operation of the engine. However, the vacuum in the venturi is not strong enough for adequate control of a vacuum actuated valve to meter the flow rate of recirculated exhaust gas.
Conventionally, an exhaust gas recirculation system has a control apparatus providing a vacuum output which is an amplification of a vacuum in the venturi of a carburetor to a valve actuator of an exhaust gas flow control valve thereby to open an exhaust recirculation conduit responsive to changes in the venturi vacuum. When, in the conventional exhaust gas recirculation system, the intake manifold vacuum increases with the venturi vacuum remains unchanged, the flow rate ratio of recirculated exhaust gas to intake air increases above an optimum value because flow rate of the exhaust gas through the recirculation conduit increases as the intake manifold increases even if the opening degree by the flow control valve is constant. Thus with the conventional exhaust gas recirculation system it is difficult to keep the flow rate of exhaust gas to an optimum ratio to that of intake air throughout all modes of operation of the engine. It is desirable that the opening degree of the flow control valve be decreased as the intake manifold vacuum increases or be increased as the intake manifold vacuum decreases so as to keep the flow rate ratio of recirculated exhaust gas to the venturi vacuum at an optimum value.